1. Field of the Invention
This invention relates to fluorescent lamps which emit radiation in the ultraviolet range of the electromagnetic spectrum. Such lamps comprise a long tubular glass envelope having an ultraviolet emitting phosphor on the inner wall thereof and having electrodes at each end of the envelope with a low pressure mercury arc discharge therebetween during lamp operation. The invention particularly concerns such lamps having a reflective film on the inner envelope wall.
Such lamps, having a phosphor that emits in the long ultraviolet or about 3200 to 4000 angstroms, have recently become useful in the treatment of psoriasis.
2. Description of the Prior Art
Fluorescent lamps having an internal reflective layer for the purpose of enhancing the forward brightness of visible light have been known for some time. Such lamps are disclosed in U.S. Pat. Nos. 2,854,600 and 3,295,003. In another version, the lamps include a longitudinal uncoated portion, a so-called aperture, facing the reflective layer as in U.S. Pat. No. 3,225,241.
Disclosed as suitable reflectors for visible light are materials having a high refractive index. A preferred visible light reflector is made of titania having a particle size about half that of the median wavelength of the visible spectrum, about 0.3 micron, although zirconia is an alternative reflective material. Other disclosed reflector materials are zinc oxide, white lead, antimony oxide, zinc sulfide, alumina and magnesia.
The three patents are particularly concerned with visible light emitting phosphors and with reflector materials suitable for reflecting visible light, although it is disclosed that alumina and magnesia can be used to reflect ultraviolet radiation. The patents state that ultraviolet reflection can be useful if the phosphor coating is too thin to absorb all the 2537 angstrom radiation from the mercury arc; then the ultraviolet reflector can reflect such 2537 radiation back with the result that the total amount of 2537 radiation that is converted to visible radiation by the phosphor is increased.
The patents disclose that magnesia is unsatisfactory as a 2537 angstrom reflecting material, because it causes heavy lamp blackening, and that titania is also a poor reflector of such radiation, the rutile form thereof reflecting only about 7%.
It is an object of this invention to provide a fluorescent lamp having an internal reflector that is efficient in reflecting radiation other than the visible radiation or 2537 radiation disclosed in the prior art.